Interest in the utilization of renewable biomass resources as fuels is growing against a background of worldwide depletion of fossil fuels, and emissions of greenhouse gases from fossil fuel combustion.
Whilst some processes can handle large volumes of biomass directly as fuels, the convenience of liquid fuels has led to the emergence of technology to convert biomass into liquids, effectively bio-oil.
Biomass as harvested typically contains impurities or compounds that can affect its processing, or cause serious equipment fouling and/or additional emissions The cleaning of biomass to remove undesirable impurities and compounds that cause processing issues and emissions is difficult to achieve economically, and is a hindrance to the development and expansion of biomass utilization processes. The impurities and undesirable compounds may be solids that cause excessive equipment wear and maintenance, or may be dissolved or occluded compounds that have similar effects and which also, in processing of combustion, generate serious emissions and pollution, or may be compounds contained within the biomass. Such compounds include fertilizer and other residues from minerals absorbed by the biomass during its growth. These compounds typically end up in ash when the biomass is combusted, but may cause fouling and high maintenance costs in conversion processes.
The conversion of biomass into either energy by combustion or other products with various processes is imperfect, and results in a certain amount of wastage, normally in the form of waste heat. The waste heat is typically lost in the form of warmed-up coolant, often warm water. One such example of biomass use as a fuel is the burning of xe2x80x9cHog Fuelxe2x80x9d by pulp and paper mills.
Pulp mills utilize the outer layers of logs, mainly bark but also small branches and leaves, also sawdust, (commonly called xe2x80x9cHog Fuelxe2x80x9d) as fuel for their boilers. When the hog fuel has been immersed in sea water, for example when the raw logs are delivered floating in sea-water, then the outer layers absorb salt water. The hog fuel produced from such sources contains substantial quantities of salt, typically 0.9% up to 2% by weight, and also sand and other debris.
The salt enters the boilers with the hog fuel, and is emitted as salt crystals or converted by chemical reaction into a variety of inorganic and organic compounds including salt cake, dioxins and furans. These materials cause corrosion of the boiler, and also constitute a major source of pollution. xe2x80x9cDioxinxe2x80x9d is a general term that describes a group of hundreds of organo-chlorine chemicals, some of the most toxic compounds known, that are highly persistent in the environment. xe2x80x9cFuransxe2x80x9d are also long-lived organo-chlorine compounds with carcinogenic and other undesirable environmental impacts. Dioxin has been described by the US Environmental Protection Agency as a serious public health threat. The International Agency for Research on Cancer (IARC, part of the World Health Organization, considers one dioxin as a Class I carcinogen. The combustion of salty hog fuel causes major emissions of dioxins.
Pulp and paper mills also produce and utilize large quantities of warm and hot water in their pulping operations. There is a net excess of such heated water, and this must go to disposal, causing thermal pollution, a waste of energy and through this additional greenhouse gases emissions.
Efforts to reduce dioxin pollution from mills to date have been directed towards capture of the dioxin, its reduction through chemical injection into the boiler where it is formed, and the use of special boiler designs or boiler operations. These methods do not avoid the formation of, or eliminate the dioxin or salt emissions, and in some cases add another pollutant from the injected materials. There is also evidence that precipitators commonly used to capture particulates in boiler flue gases provide just the right conditions to form more dioxins. The ash produced on boilers burning hog fuel containing salt also contains concentrations of dioxins. The ash containing dioxins is regarded as hazardous waste and normally goes to special lined landfills.
The method of the present invention, in contrast, inhibits or reduces, in one aspect, the formation of dioxins by removing the chlorine-containing contaminant (for example salt or NaCl) from hog fuel including mulched bark before it is burned. The method also prevents salt emissions from boiler stacks by removing the salt from the hog fuel before it enters the boilers. The salt, containing chlorine, may be returned to the sea. The invention is however broader in application than to only the solvating of salt from hog fuel in the presence of warm or hot water. Contaminants may be other than salt so long as the corresponding solvent or reactant, whether chemical or biological, required to neutralize or remove the particular contaminant is employed in the method. Consequently, it is intended herein that reference to contaminants is to be interpreted to include more than merely salt, and that reference to solvent is to include more than merely water and is to include solvent or other reactants corresponding to the particular contaminant, and therefore that reference herein to burner is to include both combustors and reactors.
This invention relates in one application of the method to the removal of salt from hog fuel by contacting the hog fuel with excess warm or hot water streams generated in a pulp mill, which streams commonly pass from the mill as effluent, or otherwise water from an outside source for use in salt extraction.
The method may employ counter current diffusion extraction equipment of any suitable configuration that will provide both mass and heat transfer. The extraction equipment may be used in a horizontal, vertical or inclined plane. In particular, the method according to the present invention may employ means of bringing hog fuel and water, into intimate contact, by using various types of extraction equipment, counter-current extraction, pipelines, fluid-beds, cyclones, cross-flow devices pulsed extractors, filters or extractors purpose-built to provide the means of mass and heat transfer, or any other contacting device or combinations of the above.
The method of the present invention is for the avoidance or reduction of dioxin creation by, for example, counter-flow washing of salt from hog fuel, and for the simultaneous recovery of waste energy. The method may be employed using:
(i) a mass flow conveyor, such as a screw conveyor, having an infeed and an opposite outfeed, providing a pre-selected number of mass and heat transfer stages so as to provide for efficient and economic operation and
(ii) a water source,
The method of comprising the steps of:
(a) feeding cold salt-laden hog fuel into said infeed of said conveyor,
(b) translating said hog fuel along said conveyor in a first direction towards said hog fuel outfeed,
(c) simultaneously feeding solvent into intimate intermingling or turbulent contact with said hog fuel in or on said conveyor, for example so as to cause a water flow in a second direction counter to said first direction in a counter-flow of said water through said translating hog fuel,
(d) extracting, salt-laden solvent following said contact and, extracting de-salted hog fuel via said hog fuel outfeed.
In one preferred embodiment the solvent, at least in part, is warm or hot effluent water. In alternative embodiments, the conveyor is a screw conveyor or may be other mass-flow conveyor means for conveying said hog fuel in said first direction from said infeed to said outfeed so long as said mass-flow conveyor means provides for, or is adapted to provide, interaction of said hog fuel (or other mineral laden mass flow) for example in counter or cross flow with said water flowing in said second direction for flushing and dissolving said minerals from said mass flow on said conveyor means and for heat exchange of energy from the water to the mass flow.
In summary, the method according to the present invention for reducing the creation of dioxins during the burning of a biomass containing a contaminant, wherein the contaminant is solvated or neutralized in the presence of a corresponding solvent, includes the steps of:
a) prior to the burning or reacting (herein collectively referred to as burning) of the biomass, conveying the biomass and the solvent into a biomass washing means,
b) washing the biomass in the solvent in the washing means, and
c) once at least a portion of the contaminant has been washed from the biomass, conveying the biomass from the washing means and into a biomass burner, and conveying the solvent from the washing means.
In one application of the method according to the present invention the biomass is hog fuel, the contaminant is salt, and the solvent is water. The method further advantageously includes the step of supplying water to the washing means by diverting an effluent stream of heated waste water, for example from a pulp or paper mill, so that the heated waste water provides the solvent. Salt is thereby reduced or no longer passes into the burner, again wherein the burner includes combustors or reactors, and wherein stack gas precipitators collecting the particulate solids operate at lower loadings as the salt is diverted from them so as to improved combustor operations. The heated waste water may be supplied into the washing means at a first temperature and the biomass may be fed into the washing means at a second temperature, where the first temperature is higher than the second temperature so that the biomass cools the waste water in the washing means. The result is a reduction in energy consumption in pulp and paper mills and biomass processors achieved through warming of incoming fuel by recovery of waste heat from the effluent stream. In such applications, advantageously the diverting of the heated waste water is diverting of heated waster water as diverted effluent from effluent from a pulp or paper mill, and wherein the pulp or paper mill is the mill in which the biomass is to be burned, so that the diverted effluent is cooled before being disposed of in a disposal step, for example, into the ocean.
In one aspect of the present invention the washing means may be a counter-current extractor such as a screw conveyor, and the washing may include counter-current intermingling of the solvent with the biomass.
In some applications the biomass may also contain solid particulate contaminants, in which case the method according to the present invention may further include the step of removing the particulate contaminants by vibration of the solvent and biomass in the washing means.
The method according to the present invention may further include providing an underflow conveyor cooperating with the counter-current extractor for the removing step so as to remove of the particulate contaminant from the washing means.
In applications where the biomass is wood knots coated in chemicals rejected from a pulping process, the method according to the present invention may include the steps of recovering the chemicals using a minimum flow of solvent, wherein solvent and chemicals pass out of a washing stage and are transported to a recovery system, whereby cleaned wood knots are made available for use as fuel.
In a further aspect of the present invention, during the washing of the biomass an additional stream of effluent containing mill sludge may also be added into the extractor, whereby fiber in the sludge is retained within the bed of biomass so as to act as a filter, thereby allowing cleaned water from the sludge to pass out of the washing stage along with the washing water.